The present invention relates to an improved process for inspecting parts which offers higher sensitivity and lower environmental toxicity and to a system for carrying out the inspection process.
Liquid penetrant inspection of parts has been used in numerous industries for decades to identify both manufactured defects and in-service damage, such as cracks, which might lead to catastrophic failure of the part under continued use. In a typical process, a clean part is immersed in a penetrant solution which migrates into any features of the part open to the surface, such as cracks, porous surfaces, etc. The part is removed from the solution, drained and wiped to remove the penetrant from the surface, leaving only that penetrant which migrated into the features of interest. A developer is then applied to bring out the trapped penetrant, and the part is examined under the appropriate light conditions such as visible if dye penetrant or ultraviolet if fluorescent. Defects are indicated by the penetrant.
One of the major factors which determines the sensitivity, i.e. ability to highlight smaller cracks and defects, of this process is the ability of the penetrant solution to seep into the features of interest. In conventional organic solvent-based systems, this ability is afforded by a combination of low viscosity and high surface tension of the solvent, which allow it to migrate into tight cracks through capillary action. Suitable solvents generally include alcohols, ethers, ketones, esters, and chlorinated hydrocarbons. Unfortunately, many of these substances, particularly the chlorinated hydrocarbons, are harmful to both people and the environment, resulting in higher costs due to protective measures/training for operators as well as hazardous waste management and disposal. If the parts being inspected are large, requiring large volumes of penetrant, these costs can be significant.
Supercritical fluid technology has been used for years in extraction processes in the food, cosmetics, and petroleum industries. Supercritical fluids offer an attractive alternative to industrial hydrocarbon and chlorofluorocarbon solvents due to their low environmental impact, high volatility, low viscosity and high solvating power in the supercritical state. Recently, this technology has made significant inroads in the precision cleaning, less than 10 micrograms of contaminant per square centimeter of surface, industry for most of the same reasons. Perhaps the most widely used of the fluids is carbon dioxide, known as SCCO2 in its supercritical state, because its critical temperature and pressure, 88xc2x0 F. and 1070 psi respectively, are easily handled by conventional process equipment and construction materials. A typical SCCO2 cleaning facility is represented in FIG. 1. As shown therein, the facility includes a liquid carbon dioxide storage tank 10, a pump 12 for transporting the liquid carbon dioxide and raising the pressure of the liquid carbon dioxide, a heater 14 for raising the temperature of the liquid carbon dioxide to a critical temperature at which it becomes supercritical carbon dioxide, a cleaning tank 16 to which the supercritical carbon dioxide is fed, a separator 18 for removing impurities and contaminants from the supercritical carbon dioxide being recycled, and a cooler 20 for lowering the temperature of the recycled supercritical carbon dioxide to convert it into a liquid, non-supercritical state. The ability to cycle between the supercritical state in the cleaning tank 16 and a subcritical state, i.e. gas-phase, in the separator 18 allows for efficient extraction of impurities and contaminants from the supercritical carbon dioxide. In the context of a cleaning system, it also facilitates any removal of the cleaning solvent. Not only does this minimize the up-front cleaning solvent cost, but also minimizes the waste generated and thus the treatment/disposal cost.
It is an object of the present invention to provide an improved process for inspecting a part for defects.
It is a further object of the present invention to provide a process as above which uses supercritical carbon dioxide (SCCO2) as the solvent phase in a liquid penetrant inspection process.
It is yet another object of the present invention to provide a system for carrying out the inspection process of the present invention.
The foregoing objects are attained by the process and the system of the present invention.
In accordance with the present invention, a process for inspecting a part to detect defects broadly comprises the steps of placing a part to be inspected in an immersion tank, introducing a mixture of a penetrant dye and a supercritical carbon dioxide solvent into the immersion tank, maintaining the part immersed in the mixture for a time sufficient for the penetrant dye to penetrate any defects in the part, removing the part from the immersion tank, and inspecting the part for the presence of any defects.
A system in accordance with the present invention broadly comprises means for forming a mixture of a penetrant dye and a supercritical carbon dioxide solvent, means for immersing a part to be inspected into the mixture for a time period sufficient to allow the penetrant dye and the supercritical carbon dioxide to penetrate defects in the part, and means for inspecting the part after removal of the part from the immersing means to detect the presence of any defects.
Other details of the process and system of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following description and the accompanying drawings wherein like reference numerals depict like elements.